The present invention relates to a process for preparing fluorine-containing copolymers having a carboxyl group which are useful for materials of fluorine resin paints.
Fluorine resins having a carboxyl group can be prepared by copolymerizing vinyl ethers having a carboxyl group and fluoroolefins. However, synthesis of vinyl ethers having a carboxyl group is very difficult. For example, an attempt to synthesize vinyl having a carboxyl group by reacting hydroxyalkyl vinyl ethers with dibasic acid anhydrides (half-esterification) results in failure, because the carboxyl group of the product immediately reacts with the vinyl group of the same product to form a ring.
Therefore, fluorine-containing copolymers having carboxyl groups may be, for example, prepared by copolymerizing fluoroolefins and vinyl ethers having a hydroxyl group, and then reacting the hydroxyl groups in the copolymer with dibasic acid anhydrides (half-esterification) (c.f. Japanese Tokkyo Kokoku No. 49323/1986). The process, however, has a defect that since the half-esterification must be carried out after copolymerization, cheap alcohols which react with the dibasic acid anhydrides cannot be used as solvents in the copolymerization.
As a result of the present inventor's intensive study for preparation of vinyl ethers having a carboxyl group, it has been found that the intramolecular ring closure reaction does not happen when hydroxyalkyl vinyl ethers are reacted with dibasic acid anhydrides in the presence of basic compounds, and that stability of the resulting vinyl ethers can be improved in the form of carboxyl salts with the basic compounds, and also that yield of copolymerization with fluoroolefins can be remarkably increased.
There is well known that electrodeposition coating compositions containing fluorine-containing polymers can give coatings having a remarkably excellent chemical resistance and weatherability in comparison with conventional coating compositions containing acrylic resins (Japanese Tokkyo Kokai Nos. 59676/1987, 127362/1987).
In case that electrodeposition coating is carried out by using compositions containing fluorine-containing polymers, however, various troubles have risen due to the presence of halogen ions. For example, according to the inventors' study, influences of halogen ions in the electrodeposition coating compositions on the electrodeposition coating process or resulting coatings are as follows: